The present invention relates to ultra-fine-grained thermoplastics in ground particulate form.
It is known to employ thermoplastics in fine to ultra-fine-grained particulate form for various purposes. An ultra-fine-grained form is desired if the thermoplastics have to be mixed as homogeneously as possible with other substances. As the thermoplastics very often represent the more expensive component in the mixture, or the thermoplastic is only an additive but not the active ingredient, or the properties of the thermoplastics have to be modified, an effort is made to employ as little as possible of the thermoplastics in the mixture in question.
Thermoplastics cannot be readily divided into a finely divided form by grinding, or if so only at great expense, and it is not possible to achieve average particle sizes smaller than 400 .mu.m. Heat is released during the grinding process and the thermoplastics, depending on their type, in particular those with adhesive or low melting point characteristics, are immediately impossible to grind down. As a means of avoiding this problem, it is known to employ cryogenic grinding processes. The material to be ground is cooled by liquefied nitrogen whereby it becomes brittle and is subjected to grinding in this cooled state. Also this process is limited with respect to particle fineness, and the yield is poor.
Precipitation processes are employed to produce thermoplastics, such as polyolefins for example, in a finer grained form than is achievable by grinding. These processes which enable thermoplastics to be produced in an ultra-fine form are multi-stage processes with long cycle times also emissions and large amounts of residual chemicals such as contaminated precipitation medium or residual solvents. Consequently, products made by precipitation methods are expensive. Furthermore, various polymers are not suitable for producing in an ultra-fine powder form using precipitation processes.